When two circuits are interconnected, flow in one will not cause flow in the other if the pressure drop in the piping common to both is eliminated. This allows us to connect small circuits within one large system. Each pump becomes a specialist. The Closely spaced tees or primary loop, keep pumps of different sizes from fighting each other in one system. The added benefit is the ability to use smaller pumps. Smaller pumps cost less, use less electricity and are easier to work on. This means each zone can run and will not be affected by the other. Different size pumps can also be used. You can look at this as light construction for plumbing. Once your primary loop is set up, multiple secondary loops tie into each zone.